Films comprising polyesters such as a polyethylene terephthalate and a polyethylene-2,6-naphthalate have been widely used for general industrial materials such as a reproduction film, an electronic material, an OHP film, a packing film, a label and a magnetic card and magnetic recording materials such as a magnetic tape. As polyester films, films comprising a polyethylene terephthalate or polyethylene naphthalate having excellent water resistance, chemical resistance, mechanical strength, dimensional stability and electric properties have been used or studied. However, the polyester films have a problem of being liable to be electrically charged. When the film is electrically charged, dust is stuck on the surfaces thereof, resulting in degradation in quality. Further, when an organic solvent is used in a film processing step, there is a risk that electric discharge from the electrically charged film may cause fire.
As a measure for solving such problems caused by electrical charge, a method comprising kneading an anionic compound such as an organic sulfonate group, metal powder, carbon powder or the like into the polyester film and a method comprising depositing a metal compound on the surfaces of the polyester film are proposed and practically used. However, such methods have problems that the transparency of the film is degraded and that processing costs are high.
Further, as other methods, various methods for forming an antistatic coating film on the surface of the film are proposed and practically used. Although low-molecular-weight antistatic agents and polymer antistatic agents are known as an antistatic agent to be contained in the antistatic coating film, they both have advantages and disadvantages. Thus, an appropriate antistatic agent is selected and used according to application purposes.
JP-A 4-28728 (the term “JP-A” as used herein means an “unexamined Japanese patent application”) discloses, as a low-molecular-weight antistatic agent, a surfactant-type anionic antistatic agent such as a long chain alkyl compound having a sulfonate group. Further, JP-A 3-255139 and JP-A 5-320390 disclose, as polymer antistatic agents, a polymer having an ionized nitrogen element in the principal chain and a sulfonate-group-modified polystyrene.
However, an antistatic coating film using a low-molecular-weight antistatic agent has a problem that a portion of the antistatic agent moves in the coating film, concentrates near the interface and moves to the opposite surface of the film, or antistatic property deteriorates as time passage. Meanwhile, an antistatic coating film using a polymer antistatic agent is not economical because it must contain the antistatic agent in a large amount or have a large thickness so as to attain a good antistatic property. Further, when film chippings unable to be used in products, e.g., film edges cut off and removed in a production process are collected and used as recycled materials for film production, coating film components contained in the recycled materials are thermally degraded during melt film production, and a significantly colored film which lacks in practicality and therefore has poor recoverability is obtained. In addition, there occur such a problem that the films have blocking therebetween and are difficult to separate from each other or the coating films are liable to be abraded, and solutions for the problems are desired.